Normally no matter how many soil and rain stacks a building has, there is only one connection--the so-called sewer service line--to the sewer main that runs under the street. The sewer service line itself runs as a rule at a slight incline, of at least 3.degree., from the building down to the sewer main. In many areas, particularly ones where the sewers were laid down a long time ago, the sewer main is not very deep. As a result when the sewer system is heavily loaded, either by a heavy rain or simply by many users employing a line intended for lighter use, the effective liquid level in the sewer can be higher than that of the in-house drain, that is the sewer main can be completely full whereas it is normally only intended to run partially full. In such situations sewage can back up through the service line into the house drain and thence into the basement with the obvious unpleasant results.
It is possible to protect against the worst effects of such a situation by the simple expedient of putting a backflow preventer or check valve in the sewer service line. Thus no sewage from outside can back up through the drain into the basement. Nonetheless since there is considerable back pressure holding the check valve closed, any waste descending the in-house waste lines will not be able to exit, and will simply rise up out of the lowermost drains in the building, creating on a small scale the same problem as with no check valve.
The disadvantages of the above-described arrangement can be overcome by a system wherein the building drain is not connected directly to the service line, but to the intake of a pump whose output is connected to an elevated sump or storage vessel itself connected via a service line to the sewer main. The storage facility itself is provided at a level which is much higher than than the highest possible liquid level in the sewer main, so that sewage cannot possibly flow back through the storage vessel. Thus waste is pumped uphill by this pump to the elevated storage facility and then flows by gravity down into the sewer main. With such a system the pump must lift all waste, even when the liquid level in the sewer main is below that in the building drain, since the path to the sewer main has a high point over which the waste must be pumped. Obviously such a system uses a considerable amount of energy to operate the pump, and requires that a very heavy-duty pump be used. Furthermore any failure of the pump will result, whether or not the drain liquid level is above or below the level in the building drain, in a backup of waste in the building.
It has also been suggested in German patent document 1,057,984 to provide a collecting sump in the building drain having a weir subdividing this sump into an upstream portion into which liquid waste flows from the building and a downstream portion connected via the sewer service line to the sewer main. A check valve is provided in the sewer service line. A float arrangement provided in the downstream portion of the collecting sump is actuated when liquid level in the downstream portion exceeds a predetermined level, as occurs when the sewer liquid level rises to close the check valve and sewage cannot flow out of the downstream portion. A pump is connected between this downstream portion and a bypass conduit in turn connected to the service line downstream of its check valve and is operated by this float arrangement to pump the waste via the bypass conduit into the service line in the event of a backup, as the pump is capable of generating sufficient pressure to overcome the back pressure in the line.
This system has the considerable advantage that the pump only operates when the effective liquid level in the sewer main is dangerously high. As a result no energy is used under normal circumstances. Furthermore it is possible to employ a relatively light-duty pump, as compared to the continuous-duty pump needed in the other above-described pump system, as the pump only operates periodically and need only be able to move the waste from a single building. Still this arrangement requires considerable extra plumbing, and normally excavation of the sewer service line in order to install it in an existing drain system. Thus the first costs are quite high.